The present invention relates to a voice coil bobbin and a speaker system.
Electric/acoustic conversion devices such as microphones and speakers are well known. One example of such devices which convert electrical signals into acoustic signals is a commonly known speaker system with a cone-shaped diaphragm (see, for example, Japanese Patent Application Laid-Open Publication No. 2005-277561).
This speaker system includes an annular magnetic circuit made up of a plate, a magnet, and a yoke, and arranged at the lower end of an annular speaker frame. High-density magnetic flux is formed in the magnetic gap between the yoke and the plate of the magnetic circuit. A voice coil is wound around a tubular voice coil bobbin such that it can vibrate along the axial direction. The front side edge of this voice coil bobbin is secured to the center hole of the cone-shaped diaphragm, and the outer periphery of the diaphragm is secured to the speaker frame via a surround.
The voice coil bobbin is required to be able to transmit a motive force which is generated in the voice coil by the input of an audio signal precisely to the cone-shaped diaphragm. Accordingly, it is required to be lightweight for high efficiency transmission, and to have a high specific modulus of elasticity so that it can prevent deformation which may be caused by the vibration, and high internal loss so that it can attenuate unwanted vibration. Paper has been commonly used as the material which satisfies these requirements because paper is low in density and has appropriate rigidity, and also it is inexpensive. Other materials have also been used, such as foils of metals (e.g. aluminum or duralumin), or resin films (e.g. polyimide).
In high power speakers, a relatively large drive current is inputted to the voice coil to generate a large motive force, and therefore the voice coil bobbin is required to have high rigidity, and also high internal loss to attenuate unwanted vibration in the high frequency range. However, these physical properties are sometimes contradictory and cannot be achieved at the same time depending on the material of the bobbin, and if the voice coil bobbin is made of a single material, it is technically difficult to satisfy both of these requirements.